While the concept of using lubrication oil as a primary coolant medium for an internal combustion engine has been studied and tested for many years, no system of this type has yet found widespread commercial acceptance. Many potential benefits, such as reduced engine manufacturing costs and increased operating efficiency and reliability, are known advantages of oil cooling systems yet few commercially available engines employ this type of cooling. In part, the failure of oil cooling to find commercial acceptance has been the result of inadequate appreciation for the heat transfer principles involved. Lacking an accurate model of such principles, designers have had to guess as to the best flow passage geometry and flow characteristics for achieving the optimal performance to cost ratio. Some design suggestions have been experimentally tested, but tests have generally shown the existance of excessive cylinder wall temperatures during engine operation. It is, thus, not surprising that a great variety of proposals have been advanced but none have been widely adopted by commercial engine manufacturers.
U.S. Pat. No. 2,085,810 issued in 1937 to Ljungstrom contains an early disclosure of a system for cooling an engine cylinder by using the lubrication oil of the engine wherein a jacket is placed around the outer surface of each cylinder wall to form an oil flow passage having a thickness which is preferably said to be in the range of 1/32 to 1/3 of an inch. In one embodiment, oil enters the flow passage formed by the jacket through an opening adjacent the mid section of the cylinder and flows generally upwardly through the jacket toward and into the engine head. By causing the oil which enters the flow passage to first contact the cylinder wall well below the hottest section of the cylinder (normally the upper region of the cylinder) a great deal of heat transfer efficiency is lost. Such inefficiency results from the fact that the greatest heat transfer occurs in a liquid medium cooling system generally by bringing the liquid at its lowest temperature into contact with the hottest portion of the structure being cooled. In the embodiment of Ljungstrom referred to above, the cooling oil is first introduced below the mid section of the liner where the oil temperature is increased before it reaches the upper-portion of the cylinder. Thus, the greatest heat removing capability of the engine oil is not concentrated on the liner region normally having the highest operating temperature.
In other embodiments illustrated in the Ljungstrom patent, oil flow through the jacket is unsymmetric with respect to the central axis of the cylinder. This lack of symmetry can lead to greater turbulence within the flow path surrounding the upper region of the cylinder where satisfactory cooling is most important. As the amount of turbulence increases so does the difficulty of constructing a theoretical model which will allow for satisfactory prediction of the heat transfer characteristics of an oil cooling system.
In U.S. Pat. No. 3,127,879 to Giacosa et al., a system for oil cooling the cylinder liners of an internal combustion engine is disclosed which includes formation of a generally cylindrical flow path around the exterior of the liner. After oil enters the flow path below the mid section of the liner, it passes upwardly toward the top of the liner for discharge through a circular channel surrounding the top portion of the liner. In order to intensify heat transfer, Giacosa et al. teaches that it is desirable to provide grooves on the outer surface of the liner to set the oil in "whirling motion". Whatever intensification in cooling is achieved by such "whirling motion", the difficulty of developing an accurate model of the heat transfer characteristics of a system involving such whirling motion is certainly increased. In the absence of an accurate model or very extensive testing, engine designers are normally forced to over design the cooling system to insure satisfactory performance. Such over design can lead to excessive power consumption by the oil flow pump which is logically the lubrication pump of the engine.
If oil cooling is to become widely accepted, it must be compatible with pre-existing engine designs and require minimal component addition and/or redesign. Yet, in the absence of an accurate theory for predicting heat transfer capacity, good engineering practice may dictate flow requirements for oil cooling systems in excess of the capacities of original equipment lubrication pumps. This situation necessitates redesign of the original equipment pump or use of an auxiliary oil cooling system pump. While extensive testing may void some of this problem, the cost of building and testing experimental internal combustion engines renders extremely impractical the trial and error approach to oil cooling system design.
In addition to the approaches illustrated in Ljungstrom and Giacosa et al., other types of oil cooling for internal combustion cylinders are disclosed in U.S. Pat. Nos. 2,944,534 to Hodkin and 3,687,232 to Stenger and in British Pat. No. 2,000,223 to Brighigha. The Hodkin and Brighigha patents disclose cylinder wall oil cooling where the oil flow path forms a helical pattern around the central axis of the cylinder wall. Because the cooling oil contacts only a portion of the outer surface of the cylinder in these designs, excessive temperature in certain areas of the liner are more likely to occur than with systems in which the entire outer surface of the liner is contacted by the cooling oil. Moreover, these references fail to suggest a predictive model for achieving the best possible performance to cost ratio in oil cooling system design and, therefore, do not avoid the design problems noted above. The Stenger patent discloses a complex flow geometry for oil cooling the walls of an engine cylinder but again fails to disclose a mechanism for predicting, and optimizing thereby, the heat transfer characteristics of an oil cooling system.
U.S. Pat. No. 4,108,135 to Lubis discloses an arrangement for external oiling of cylinder liners by providing a very small clearance between the cylinder liners and the surrounding engine block through which oil "seeps" downwardly from an annular oil supply channel provided near the top of the liner. Although Kubis suggests supplying lubrication oil near the top of a cylinder liner, the oil so supplied is not used as a coolant medium for removing heat but serves only to improve the transfer of heat into the surrounding portion of the engine block. Kubis thus fails to address the question of how best to design a cooling system employing lubrication oil to cool the cylinder walls of an internal combustion engine.
Another crucial aspect in designing an optimal oil cooled liner involves the manner by which the liner is mounted within the engine. As noted in a copending application, Ser. No. 959,702 filed Nov. 13, 1978, now U.S. Pat. No. 4,244,330, and assigned to the same assignee as this application, certain advantages result from placement of the liner stop (that is the radial shoulder which holds the liner in a fixed axial location within a cylinder bore) closer to the innermost portion of the liner. Such advantages include improved combustion gas sealing and reduced engine block cracking which results from utilization of the greater natural resilience of the liner. Reduced production costs also result from the use of inwardly positioned liner stops since the close manufacturing tolerances required with "top stop" liner designs can be relaxed. Normally, the use of bottom or mid stop liner designs introduces many complications when the liner is of the more conventional water cooled type. However, an oil cooled liner does not need to provide high integrity in the inner (or lower) oil coolant seal between the engine cylinder and liner since oil which leaks through the inner seal will merely enter the crankcase and thus will return to the oil circuit of the engine. Some prior art oil cooled liners such as disclosed in U.S. Pat. No. 3,127,879 to Giacosa et al., and U.S. Pat. No. 2,085,810 to Ljungstrom noted above, include bottom stop designs but fail to suggest any technique for exploiting the advantages of bottom stop liners to achieve better combustion gas sealing.
In summary, the prior art describes a great variety of oil cooling systems for internal combustion engines but fails to describe an oil cooling system having sufficiently optimal passage geometry and fluid flow characteristics to be a viable option for commercial engine manufacturers.